Windows native VPN

[Brian-BBCM]

Hello,

We have a use case in which we need to use the native Windows VPN client to connect to the Fortigate. I was able to successfully configure the VPN gateway and device tunnel. However, the issue now is that the Windows native client defaults to weak cryptography using DH group 2. If I try to specify anything stronger, it breaks the VPN connection.

I would like to add the following to my profile.xml file. When I add these lines, the VPN immediately disconnects and the only way to reconnect is to remove these lines and regenerate the device tunnel.

  <CryptographySuite>
   <CipherTransformConstants>AES256</CipherTransformConstants>
   <EncryptionMethod>AES256</EncryptionMethod>
   <IntegrityCheckMethod>SHA384</IntegrityCheckMethod>
   <DHGroup>Group14</DHGroup>
<PfsGroup>PFS2048</PfsGroup>
<AuthenticationTransformConstants>SHA256128</AuthenticationTransformConstants>

For reference, I have these set up, respectively:

Phase 1:

aes256-sha512

aes256-sha384

DH groups 14 & 2

Phase 2:

aes256-sha1

aes256-sha384

aes256-sha256

[Markus_M]

Hey Brian,

if you can change the DH group and other settings that'd be best. FGT will accept much stronger ciphers, however whatever the client offers and the server - one set of ciphers must match on both client and server.

Example on my device:

set proposal aes256gcm-prfsha256

set dhgrp 21

Best regards,

Markus

[Brian-BBCM]

Thank you, but I have triple checked and confirmed there are matching proposals on both sides. If there was a mismatch, I would have found it by now. The VPN appears to disconnect if I specify any settings at all.

[Brian-BBCM]

I believe I found the solution though I cannot pinpoint exactly what I did. After trying various combinations of settings for hours, I was able to get it to connect using DH group 14 and AES256 encryption. It seemed to come online after I enabled Group 14 on PFS. Ironically it now seems that PFS is the only thing not working correctly as I see this entry in the Fortigate debug log: PFS is disabled.

As said above, I have enabled DH group 14 on PFS for phase 2. However the native Windows client only has these options available.

PFS1
PFS2
PFS2048
ECP256
ECP384
PFSMM
PFS24

Does anyone know which DH groups these correspond to? I would have thought PFS2048 is DH group 14 but it is not working correctly.

Thanks again.

[Markus_M]

Hi Brian,

glad it got better at least. For reference I collected this over time in my IT pile:

Diffie-hellman groups	Modulus
Diffie-Hellman Group 1	768-bit MODP group
Diffie-Hellman Group 2	1024-bit MODP group
Diffie-Hellman Group 5	1536-bit MODP group
Diffie-Hellman Group 14	2048-bit MODP group
Diffie-Hellman Group 15	3072-bit MODP group
Diffie-Hellman Group 16	4096-bit MODP Group
Diffie-Hellman Group 17	6144-bit MODP Group
Diffie-Hellman Group 18	8192-bit MODP Group
Diffie-Hellman Group 19	256-bit elliptic curve group
Diffie-Hellman Group 20	384-bit elliptic curve group
Diffie-Hellman Group 24	2048-bit, 256 bit subgroup

FortiGate debug will help you to translate what the client is offering so you can guess better what you want to set.

Debug on the FortiGate console is as follows:

diag debug console timestamp enable

diag debug app ike -1

diag debug enable

Then connect again.

Best regards,

Markus

[Markus_M]

The ike debug will tell you if it has a problem with the proposals, so please post that output.

Example on my lab:

ike 0:forti.lab-VPN:422:forti.lab-VPN:234003: responder selectors 0:192.168.40.8:0->0:0.0.0.0/0.0.0.0:0
ike 0:forti.lab-VPN:422:forti.lab-VPN:234003: my proposal:
ike 0:forti.lab-VPN:422:forti.lab-VPN:234003: proposal id = 1:
ike 0:forti.lab-VPN:422:forti.lab-VPN:234003: protocol id = IPSEC_ESP:
ike 0:forti.lab-VPN:422:forti.lab-VPN:234003: PFS DH group = 14
ike 0:forti.lab-VPN:422:forti.lab-VPN:234003: trans_id = ESP_AES_CBC (key_len = 128)
ike 0:forti.lab-VPN:422:forti.lab-VPN:234003: encapsulation = ENCAPSULATION_MODE_TUNNEL
ike 0:forti.lab-VPN:422:forti.lab-VPN:234003: type = AUTH_ALG, val=SHA1
ike 0:forti.lab-VPN:422:forti.lab-VPN:234003: trans_id = ESP_3DES
ike 0:forti.lab-VPN:422:forti.lab-VPN:234003: encapsulation = ENCAPSULATION_MODE_TUNNEL
ike 0:forti.lab-VPN:422:forti.lab-VPN:234003: type = AUTH_ALG, val=SHA1
ike 0:forti.lab-VPN:422:forti.lab-VPN:234003: incoming proposal:
ike 0:forti.lab-VPN:422:forti.lab-VPN:234003: proposal id = 1:
ike 0:forti.lab-VPN:422:forti.lab-VPN:234003: protocol id = IPSEC_ESP:
ike 0:forti.lab-VPN:422:forti.lab-VPN:234003: PFS DH group = 14
ike 0:forti.lab-VPN:422:forti.lab-VPN:234003: trans_id = ESP_AES_CBC (key_len = 128)
ike 0:forti.lab-VPN:422:forti.lab-VPN:234003: encapsulation = ENCAPSULATION_MODE_TUNNEL
ike 0:forti.lab-VPN:422:forti.lab-VPN:234003: type = AUTH_ALG, val=SHA1

Best regards,

Markus